In firearms terminology, the receiver is the part of the firearm that houses
the operating parts of the gun. In most cases, it is made up of forged or
stamped metal, and the moving parts reciprocate and cycle within it.

In strictly legal terms, the receiver is the actual firearm itself, and as such
it is the controlled part (without which operating is impossible). Generally,
the law views the receiver as that part of a firearm housing that has the
serial number upon it. Thus, in the case that a firearm has multiple
receivers, the legally controlled one is the one that is serialized.